The hydrogenase and the components of the nitrogenase from the bacteroid form of Rhizobium japonicum are being purified so that their properties can be investigated in detail. The nature of hydrogenase is of interest because it has the potential for recovering energy that otherwise might be lost fron N2-fixing systems by dissipation as H2. There are two hydrogenases in the blue-green alga Anabaena 7120. One hydrogenase is reversible and the other supports only H2 uptake. The "uptake" hydrogenase is much more concentrated in the heterocysts than in vegetative cells. These hydrogenases are being purified. All isolated nitrogenase systems function anaerobically, but in vivo they operate at various oxygen concentrations. We are studying the effect of the PO2 on N2 fixation, growth and efficiency of substrate utilization by Rhodospirillum rubrum, Bacillus polymyxa and other microorganisms. The nitrogenase reductase as isolated from R. rubrum is inactive, but it can be activated by incubation with a factor (activating gactor) eluted from R. rubrum chromatophores with salt solution. The activating factor is being purified so that its chemical nature and its mode of action can be established.